Covalent Bonds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
What is a Covalent Bond? Covalent Bond A strong bond formed when two atoms share a pair of electrons. Each atom contributes one electron to the shared pair. Covalent bonding occurs between non-metal atoms .
Instead of transferring electrons (like in ionic bonding), the atoms share electrons to achieve stable electronic configurations. Why Do Atoms Form Covalent Bonds? Non-metal atoms have nearly full outer shells (typically 5, 6, or 7 electrons) They need to gain electrons to achieve a full outer shell (stable noble gas configuration) When two non-metals meet, neither can easily give up electrons The solution: share electrons so both atoms can fill their outer shells Shared electrons count towards both atoms' outer shells Key Concept In covalent bonding, atoms share electrons.
In ionic bonding, atoms transfer electrons. Simple Examples of Covalent Bonds Example 1: Hydrogen Molecule (H₂) Electronic configuration: Hydrogen has 1 electron Needs: 1 more electron to fill outer shell (stable like helium: 2 electrons) H • + • H → H : H or H-H Each H shares its electron with the other H Both atoms now have 2 electrons (full outer shell) Each hydrogen atom contributes 1 electron They form 1 shared pair = 1 covalent bond Both atoms now have a full outer shell (2 electrons) Example 2: Chlorine Molecule (Cl₂) Electronic configuration: Chlorine has 7 electrons in outer shell (2.8.7) Needs: 1 more electron to fill outer shell (8 electrons, like argon) Cl • + • Cl → Cl : Cl or Cl-Cl Each Cl shares one electron with the other Cl Both atoms now have 8 electrons in outer shell Each chlorine atom contributes 1 electron They form 1 shared pair = 1 single covalent bond Both atoms achieve stable noble gas configuration (8 electrons) Example 3: Water Molecule (H₂O) Oxygen: 6 electrons in outer shell (2.6) - needs 2 more Hydrogen: 1 electron - needs 1 more H | H - O (simplified) Oxygen shares electrons with 2 hydrogen atoms O has 8 electrons in outer shell (full) Each H has 2 electrons (full) Oxygen forms 2 covalent bonds (one with each H) Oxygen now has 8 electrons in its outer shell Each hydrogen has 2 electrons in its outer shell All atoms achieve stable configurations Single, Double, and Triple Covalent Bonds Single Covalent Bond Single Bond One shared pair of electrons between two atoms.
Represented by a single line: - Examples: H-H, Cl-Cl, H-Cl Double Covalent Bond Double Bond Two shared pairs of electrons between two atoms. Represented by a double line: = Example: Oxygen molecule (O₂) O = O Each oxygen: 6 electrons in outer shell Each needs 2 more electrons They share 2 pairs = double bond Both now have 8 electrons Other examples: CO₂ has two double bonds: O=C=O Triple Covalent Bond Triple Bond Three shared pairs of electrons between two atoms.
Represented by a triple line: ≡ Example: Nitrogen molecule (N₂) N ≡ N Each nitrogen: 5 electrons in outer shell Each needs 3 more electrons They share 3 pairs = triple bond Both now have 8 electrons Bond Type Shared Pairs Representation Examples Single 1 pair (2 electrons) - H₂, Cl₂, CH₄, H₂O Double 2 pairs (4 electrons) = O₂, CO₂, C₂H₄ Triple 3 pairs (6 electrons) ≡ N₂, C₂H₂ Dot and Cross Diagrams for Covalent Bonds We use dot and cross diagrams to show how electrons are shared in covalent bonds: Electrons from one atom are shown as dots (•) Electrons from the other atom are shown as crosses (×) Only outer shell electrons are shown Shared pairs are shown in the overlap region between atoms Example: Hydrogen (H₂) H• ×H → H • × H The shared pair (one • and one ×) is between the atoms Example: Water (H₂O) × × H• O ×H × × Oxygen has 6 outer electrons (shown as ×) Each H has 1 electron (shown as •) Two shared pairs shown where atoms overlap Example: Methane (CH₄) H• | H•-C-•H | H• Carbon has 4 outer electrons Forms 4 single bonds with 4 hydrogen atoms All atoms achieve full outer shells Exam Tip When drawing dot and cross diagrams: (1) Count outer electrons for each atom, (2) Show how many electrons each needs to gain, (3) Draw shared pairs in overlap regions, (4) Check all atoms have full outer shells (2 for H, 8 for others).
Common Molecules and Their Bonds Molecule Formula Bonds Structure Hydrogen H₂ 1 single bond H-H Oxygen O₂ 1 double bond O=O Nitrogen N₂ 1 triple bond N≡N Water H₂O 2 single bonds H-O-H Ammonia NH₃ 3 single bonds H-N-H with one H below Methane CH₄ 4 single bonds Tetrahedral shape Carbon dioxide CO₂ 2 double bonds O=C=O Hydrogen chloride HCl 1 single bond H-Cl How Many Bonds Does Each Element Form?
The number of covalent bonds an atom forms depends on how many electrons it needs to fill its outer shell: Element Outer Electrons Electrons Needed Number of Bonds Examples Hydrogen (H) 1 1 1 bond H₂, H₂O, NH₃, CH₄ Oxygen (O) 6 2 2 bonds H₂O, O₂ (double bond) Nitrogen (N) 5 3 3 bonds NH₃, N₂ (triple bond) Carbon (C) 4 4 4 bonds CH₄, CO₂, C₂H₆ Chlorine (Cl) 7 1 1 bond Cl₂, HCl Remember H = 1 bond, O = 2 bonds, N = 3 bonds, C = 4 bonds.
This pattern helps you work out mol…
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